The present invention relates to an inkjet printhead and a method of inkjet printing. In particular, the invention relates to a thermal ink jet printhead and a method of thermal ink jet printing.
Inkjet printers print dots by ejecting very small drops of ink onto a print medium, generally paper, and typically include a movable carriage that supports one or more printheads each having ink ejecting nozzles (or orifices). The carriage traverses over the surface of the print medium, and the nozzles are controlled to eject droplets of ink at appropriate times pursuant to commands from a microcomputer or other controller, wherein the timing of the ejection of the ink drops is intended to correspond to the pattern of pixels of the image being printed.
The printheads of thermal ink jet printers include one or more ink reservoirs and a nozzle plate having an array of ink ejecting nozzles, a plurality of ink vaporization chambers in communication with the respective nozzles, and a plurality of heating resistors, known as “firing resistors”, within the vaporization chambers and opposite the ink ejecting nozzles which are spaced therefrom by the vaporization chambers.
To print a single dot of ink, an electrical current from an external power supply is passed through a selected heating resistor. Localised heat transfer from the resistor to a defined volume of ink within the vaporization chamber vaporizes said volume of ink and causes it to expand thereby causing a droplet of ink to be ejected through the associated orifice onto a print medium. Properly arranged nozzles form a dot matrix pattern.
Also provided is a control unit, connected with the firing resistors for selectively activate the same. When a selected resistor is to be activated, and thus when the associated nozzle is to eject an ink droplet, the control unit generates a command signal, which causes a current to flow through the selected resistor, thereby heating the resistor. Such a command signal to the firing resistor is also referred to as the fire signal, which is generally provided as a pulse signal.
There is a trend in thermal inkjet technology to increase the number of nozzles constructed on a single printhead as well as to increase the firing rate of those nozzles. As the number of nozzles increases, the number of external connections increases, thereby increasing the hardware complexity of the printhead and of the wiring structure thereof. Multiplexing can be implemented in which some of the connections are shared by the ink firing resistors on a time division basis so as to reduce the number of interconnections to the printhead.
In a known multiplexing scheme, a gating transistor is electrically connected to each firing resistor and current to the resistor flows only when its associated gating transistor is selected.
U.S. Pat. No. 6,543,882 discloses a printhead in which firing resistors are arranged into a plurality of matrices of rows and columns. In more detail, the cited patent discloses a dynamic memory based ink firing cell including a firing resistor and a dynamic memory circuit. Each firing resistor is associated to a drive transistor. The gate of the drive transistor forms a storage node capacitance that functions as a dynamic memory element that stores resistor energising or firing data.
Some important parameters generally need to be taken into account when assessing the overall quality of a printhead and thus when designing a printhead. A first relevant parameter is the maximum firing rate, which is the firing rate, also referred to as “working frequency”, at which each resistor can be successively energized. In other words, it takes some time to generate an ink drop, to eject the ink drop and to be ready to start a new generation of an ink drop from the same nozzle by heating the same resistor. Other parameters that can determine the functionalities of a printhead are the time between successive firings of different resistors (firing cycle) and the number of resistors that can be fired in a firing cycle.
The Applicant has observed that as long as the fire signal is in the high state (i.e., during the fire pulse), the printing data in each firing cell of the addressed matrix needs to be stable until the required pattern has completed the printing operation. It follows that the working frequency of a printhead having a dynamic memory element can be low, since each firing cell can not be loaded with new data until the fire pulse is over.
Applicant has further observed that when using a dynamic memory element in the firing cell, the printing data must be present up to the end of the firing step (typically corresponding to the printing operation): Therefore, before storing new data in a storage node, which functions as a dynamic memory element, it is necessary to wait until the end of the firing step which involves the drive transistor and the associated firing resistor of a firing cell.
The Applicant has realized that if the time during which a first firing step is carried out is employed for the loading of at least a portion of new data necessary to perform a second firing step, successive to the first firing step, the overall time of the printing operation can be considerably reduced. In other words, if during a firing step, i.e., during transferring of the pulse energy to the firing cells, loading of the energizing binary data for the next firing step is executed, an increase of the printhead working frequency can be achieved.
In particular, since a time-consuming step is that of receiving printing data from the control unit, such a step can be advantageously carried out while a first firing pulse is being transmitted from the control unit to the firing cell(s), thus saving time between the transmission of the first firing pulse and the transmission of a second (successive) firing pulse.